The present invention relates to a method and apparatus for forming a thin film on the surface of a plastic member, a glass member, or the like. In particular, the invention relates to a method and apparatus for forming thin films, having constant optical properties with high reproducibility, that are suitable for use in, for example, forming an antireflection film on a lens for eyeglasses.
In the formation of thin films on the surfaces of plastic or glass members, it is desirable to form thin films having the same quality fully automatically with high reproducibility. As an attempt to satisfy the above requirement, Japanese Unexamined Patent Publication No. Hei. 3-50501 discloses a method and apparatus in which the value of current supplied to an electron gun is monitored, the actual current data is compared with preset reference data, and, when a deviation occurs between the actual and reference data, the value of current to be supplied to the electron gun is corrected and controlled in accordance with the deviation.
Furthermore, Japanese Unexamined Patent Publication No. Sho. 61-147874 discloses a method and apparatus in which the thickness of an evaporated film is detected by a quartz oscillator method and the value of current to be supplied to an electron gun is corrected and controlled. In the quartz oscillator method, a film thickness is calculated by sticking a thin film to a quartz oscillator and detecting a variation of the inherent frequency that is proportional to a variation of its mass. Still further, Japanese Unexamined Patent Publication No. Hei. 7-180055 discloses a control method in which the time necessary for film formation is managed so as to be equal to a target time necessary for film formation by correcting a current value of an evaporation source by using a quartz film thickness meter and the evaporation rate is thereby made constant.
However, in the methods disclosed in Japanese Patent Publication Nos. Hei. 3-50501 and Hei. 7-180055, it is difficult to cope with a phenomenon that the degree of evaporation of an evaporating substance varies as the film forming apparatus is used repeatedly. In particular, for thin films for optical products such as an antireflection film for eyeglass lenses (in optical products, tolerances of errors relating to the quality of a thin film are small), these methods are not suitable for forming thin films having the same quality fully automatically with high reproducibility. This is because, in the methods disclosed in Japanese Patent Publication Nos. Hei. 3-50501 and Hei. 7-180055, even if the current value of the electron gun is controlled as correctly as possible, a thin film formed may be much different from an intended film when any of a number of parameters, e.g., the state of the filament of the electron gun, the substrate temperature, the vacuum conditions, etc., differ from a set condition.
The methods disclosed in Japanese Patent Publication Nos. Sho. 61-147874 and Hei. 7-180055 in which the value of current supplied to the electron gun is corrected and controlled by using a quartz oscillator have the following problems. In the quartz oscillator method, the measured physical quantity is a variation of frequency that is caused by a variation of the mass of film that is stuck to the quartz oscillator. Therefore, the quantity that is measured through the frequency is the mass of a film and not the optical film thickness. The optical film thickness is determined by the product of the geometrical thickness and the refractive index of the film and is not uniquely determined by the mass.
As described above, the quartz oscillator method measures the mass that is not in a unique relationship with the optical film thickness. Therefore, to determine an optical film thickness from a mass value, it is necessary to make a calculation by determining a proper conversion coefficient according to empirical rules. However, since such a conversion coefficient depends on various conditions such as film formation conditions, a difference in such conditions directly leads to a conversion error. Therefore, not only is it difficult to determine a proper conversion coefficient but also, in principle, there are no large differences between the methods concerned and the above methods in which the current of the electron gun is controlled.
In addition, the methods concerned are very easily affected by electrical noise because a quartz oscillator is disposed in a film forming chamber, the same film as an intended film is formed thereon, and the frequency of the quartz oscillator is measured electrically as a faint electrical signal. However, as many noise sources exist in the film forming chamber, it is no exaggeration to say that the chamber is filled with electrical noise sources. Therefore, there is another problem that, in reality, it is not necessarily easy to detect a signal by eliminating these noises.
The present invention has been made to solve the above problems, and an object of the invention is to provide a thin film forming method and apparatus that can automatically form thin films having constant optical properties with high reproducibility by controlling, during a film forming process, the amount of film forming material supplied (such as by vaporization) so that a measured value that uniquely depends on the instantaneously varying optical film thickness becomes or approaches a standard value.
As a means for solving the above problems, the inventors have provided a thin film forming method for performing film formation by vaporizing a film forming material and depositing a thin film of the material on a surface of an object, comprising the steps of:
measuring a thickness of the thin film deposited with an optical film thickness meter that measures the thickness of the thin film by measuring, a quantity of light transmission or reflection obtained when a prescribed light is applied to the object on which the thin film is formed; and
controlling an amount of the film forming material vaporized so that a value of the quantity of light transmission or reflection measured at each time point by the optical film thickness meter becomes equal or approximately equal to a standard light quantity value.
The inventors have also provided a thin film forming apparatus having film forming means for causing a film forming material to fly in a film forming chamber and depositing it on a surface of a subject of film formation, comprising:
an optical film thickness meter for measuring a thickness of a thin film by obtaining a quantity of light transmission or reflection when a prescribed light is applied to an object on which the thin film is formed;
storing means for storing (a) standard values of the quantity of light transmission or reflection at given points in time in a film forming process measured by the optical film thickness meter when a prescribed light is applied to an object on which a film is formed, or (b) standard values of the variation of the quantity of light transmission or reflection at given points in time in a film forming process measured by the optical film thickness meter when a prescribed light is applied to an object on which a film is formed; and
control means for controlling an amount of a film forming material that is vaporized by the film forming means so that a quantity of light transmission or reflection measured at a given time point, or a variations in the quantity of light transmission or reflection at a given time point becomes equal to or approximately equal to standard values at corresponding points in time.
As a means for solving the above problems, the inventors have also provided a thin film forming method for forming a thin film by vaporizing a film forming material with an electron gun and depositing the material on a surface of an object, comprising the steps of:
measuring a thickness of the thin film with an optical film thickness meter that measures a thickness of the thin film by measuring a quantity of light transmission or reflection that depends on the thickness of the film when a prescribed light is applied to the object on which the thin film is formed;
measuring, at intervals of time, light quantity transmission or reflection values from a start to an end of a standard film formation with the optical film thickness meter, and thereby generating, standard light quantity value data of light quantity values versus film formation time as measured from the start of the film formation; and
measuring actual light quantity values of transmission or reflection from a start to an end of a film formation, and controlling power supplied to the electron gun so that the light quantity value becomes equal or approximately equal to a standard light quantity value at a corresponding time point of the standard light quantity value data.
The inventors have also provided a thin film forming apparatus which performs film formation by causing a film forming material to vaporize by using an electron gun and depositing it on a surface of a subject of film formation, comprising:
an optical film thickness enter for measuring a thickness of a thin film by utilizing a fact that a transmission or reflection light quantity that is obtained when a prescribed light is applied to a subject of film formation on which the thin film is formed depends on the thickness of the thin film;
storing means for storing standard light quantity value data as pairs of a light quantity value and a film formation time as measured from a start of film formation that are generated by performing the film formation and measuring at intervals transmission or reflection light quantity values from the start to an end of the film formation with the optical film thickness meter; and
control means for comparing, during film formation, a light quantity value that is measured at each time point by the optical film thickness meter with a standard light quantity value at a corresponding time point of the standard light quantity value data stored in the storing means, and for controlling power to be applied to the electron gun so that the actual and standard light quantity values become equal or approximately equal.